How many users can I create?

You can create as many users as you want. You pay per activated user per month.

Example: You have 3 active users at the start of the month, during the month you activate 1 additional user.

On your next invoice, you will be billed for a full month for the three active users. For the new user you will be billed for the number days it has been active.

Related articles:

How do I create a new Office 365 user?
When do I pay for a user?